The invention relates to a grid sensor with wire electrodes for measuring the electroconductivity of a flow medium in the cross-section of a pipeline. The invention is used in particular when the flow medium flows under high pressure and at high temperatures.
Johnson [U.S. Pat. No. 4,644,263] first describes a grid sensor that comprises electrically conducting metal wires that are under tension in the conduit cross-section and are electrically insulated from the conduit wall. Two planes of such electrode grids are installed in the conduit immediately behind one another such that the individual wires intersect at an angle of 90 degrees without touching one another. The wires are connected to an electronic circuit that connects the electrodes of the first plane via a multiplexer successively to a voltage source and then switches the electrodes of the second plane via a second multiplexer to a voltage-sensitive detector. The voltages that occur on the electrodes of the second plane are compared to a threshold value. If the latter is exceeded, it is assumed that a conductive medium, e.g. liquid, is currently at the corresponding point of intersection for the electrodes of both planes. Using the multiplexer, all available points of intersection are queried and the number of those at which the conductive phase is detected is determined. A measure for the mean volumetric portion of the conductive phase in the flow cross-section is obtained with respect to the total number of points of intersection. No information is provided about attaching or passing the electrodes through any conduit walls.
DD 282 376 A7 describes a conductive measuring cell with two electrode planes in which one plane comprises lamella-like plates arranged in the direction of flow that have slit-shaped apertures distributed uniformly across the length. A wire plane is arranged rotated by 90°. The flow is laminarized due to the lamellae added in the direction of flow and is thus heavily influenced. This patent does not provided detailed structural configuration and employment limits.
Patent DE 196 49 011 A1 describes grid sensors that comprise a circuit board, and a grid plane is soldered to each of its sides. Moreover, this patent describes a grid sensor that possesses bars with a lens-shaped cross-section rather than electrode wires. This shape of the electrodes is intended to offer greater resistivity to mechanical stresses and is thus intended for industrial use, however at the same time it is meant to assure the least possible effect on the flow and the lowest possible loss in pressure. However, it is disadvantageous that the effect on the flow is significantly higher when the bars are used than with sensors having wire electrodes. No detailed information regarding the mechanical configuration of the sensor is provided.
It is a disadvantage of the known arrangements of sensors with wire electrodes that they are not suitable for use at high pressures and high temperatures. In addition, exchanging the electrodes is difficult or even in some cases not possible for most of the known arrangements.